The liquid crystal device has becomes used as various home appliances, measuring instruments, panels for automobiles, ward processors, electronic notebooks, printers, computers, and televisions, including clocks and calculators. Typical examples of a liquid crystal system include TN (twist nematic) type, STN (super twist nematic) type, DS (dynamic photo scattering) type, GH (guest host) type, IPS (in-plane switching) type, OCB (optical compensation birefringence) type, ECB (electric control birefringence) type, VA (vertical alignment) type, CSH (color super homeotropic) type, or FLC (ferroelectric liquid crystal). Moreover, as a driving system, multiplex driving is more general than conventional static driving, and a simple matrix system, recently, an active matrix (AM) driven by TFT (thin-film transistor) or TFD (thin-film diode) or the like has become a main stream.
In these display systems, IPS type, ECB type, VA type, CSH type, and so forth, are different from TN type or STN type, and have a characteristic of using a liquid crystal material having a negative dielectric anisotropy (Δ∈). Among them, in particular, the VA-type display by AM driving is now most expected in application to a high-speed display device requiring a wide viewing angle such as television.
Low-power driving, high-speed response, and wide operation temperature range are required for the liquid crystal material used in a display system such as VA type. That is, it is required that the dielectric anisotropy is negative and its absolute value is large and that the viscosity is low and that a transition temperature (Tni) of nematic phase-isotropic liquid phase is high. Moreover, from setting of Δn×d that is the product of refractive anisotropy (Δn) and cell gap (d), the refractive anisotropy of the cleaning liquid material is required to control to be in an appropriate range with being adapted to the cell gap. Moreover, for realizing high-speed response, the cell gap of the display device is made to be small. However, narrowing of the cell gap has been limited from such restriction as described above. For improving a response speed without change the cell gap, it is effective to use to a liquid crystal composition having a low viscosity. In the case of applying the liquid crystal device to television and so forth, high-speed response is emphasized and therefore, it has particularly been required to develop a liquid crystal composition having a low viscosity.
As the liquid crystal material having a negative dielectric anisotropy, there are disclosed the following liquid crystal compounds having a 2,3-difluorophenylene skeleton (see, Japanese Unexamined Patent Application, First Publication No. Sho 60-199840, and Japanese Unexamined Patent Application, First Publication No. Hei 2-4725):
wherein R and R′ represents an alkyl group or an alkoxy group having 1 to 10 carbon atoms. Furthermore, these patent documents include compounds having a 1-hydroxy-2,3-difluoro-4-substituted benzene skeleton as a basic skeleton of a liquid crystal compound constituting the invention of the present application. However, various compounds are described in the reference documents and there is no concrete disclosure about a compound having an alkenyl group in the side chain, and the liquid crystal composition having a negative dielectric anisotropy using the described compound does not come to realize a sufficiently low viscosity in the liquid crystal composition requiring high-speed response such as liquid crystal television.
On the other hand, there is disclose of the compound having 1-hydroxy-2,3-difluoro-4-substituted benzene skeleton as a basic skeleton of a liquid crystal compound constituting the invention of the present application (see, Japanese Unexamined Patent Application, First Publication No. Hei 8-104869, Japanese Unexamined Patent Application, First Publication No. 2000-96055, European Patent Application No. EP0474062 (page 14)). However, there is no concrete description of a liquid crystal composition using a compound having an alkenyl group in a side chain, and there is no specific disclosure about what compound should be used with the compound for lowering the viscosity of the liquid crystal composition.
Moreover, the liquid crystal compound having a 2,3-difluorohydroquinone skeleton has already been disclosed (see, Published Japanese Translation No. Hei 2-503568 of the PCT Application, and German Patent Application No. DE3906058), and also there is disclosure of a liquid crystal using the compound. However, the compound has a hydroquinone skeleton and therefore it is considered that the compound cannot be used for the active matrix in voltage retention (see, Hiroshi, Numata, Monthly Display, Vol. 4, No. 3, pp. 1-7, (1998) (page 5, table 4)), and thus the development of a low-viscosity liquid crystal composition for VA using the compound has been delayed.
Accordingly, it has been required to develop a liquid crystal composition having a negative dielectric anisotropy and a low viscosity.